Self-healing coatings for refractory metals and method for applying the same



. Patented June 12, 1962 ice 3,038,817 SELF-HEALING COATINGS FOR REFRACTORY METALS AND METHQD FOR APPLYING THE SAME This invention relates to coatings for refractory metal articles such as molybdenum, niobium and vanadium to enhance their resistance to high temperature and corrosive atmospheres. More particularly, the invention relates to the manufacture of a self-healing coated refractory metal article and the method for applying the self-healing coating to the article.

The major obstacle to a wider utilization of molybdenum articles in high temperature, high strength applications is its complete lack of resistance to oxidation at temperatures above l000 F. This lack of resistance to oxidation is due to the formation of a highly volatile molybdenum trioxide which prevents the formation of a protective oxide on the metal surface. At about 1450" F., the evaporation of the molybdenum trioxide is approximately equivalent to its rate of formation; and at 1800* F. in slowly flowing air, the surface of the molybdenum recedes at a rate of 0.02 to 0.05 inch per hour, as compared with a rate of 0.00073 inch per hour for satisfactory iron-chrome-nickel alloys.

Under these circumstances, molybdenum is useless for high temperature applications unless some form of surface protection is developed. Several approaches to this problem have been undertaken and many patents have been issued encompassing the protection of molybdenum or its alloys. All of these patents, however, involve alloys or coatings that are not self-healing. That is, the coatings will not repair themselves if a break occurs therein. Consequently, if a pin hole or other break occurs in the coating, then the molybdenum volatilizes at high temperatures. All that remains is the shell of the coating with the main structure escaping by volatilization of the trioxide through the break in the coating. For a coating on a refractory metal article to be successful, it must have, in addition to a low vapor pressure at high temperatures, adequate resistance to oxidation, thermal shock, impact and erosion, and it should be able to Withstand a minimum amount of creep strain without fracture. A self-healing coating which will repair itself Whenever a break occurs therein can accomplish all of these requirements.

The many oxidation resistant coatings for molybdenum and other refractory metals that have been studied in an attempt to solve the problem of oxidation include silicon plus nitrogen, silicon plus boron, silicon plus aluminum, aluminum with any combination of boron-silicon-titaniumzirconium, nickel plus aluminum, nickel plus chromium, nickel phosphide, chromium plus platinum, gold, vitreous coatings, vitreous coatings with cobalt, and graphite or carbon. None of these coatings, however, have provided substantial protection against oxidation since all of the aforesaid requirements have not been fulfilled. That is, the previous known coatings were deficient in adequate resistance to oxidation, thermal shock, irn act and erosion and, in general, were not self-healing.

Accordingly, it is a primary object of this invention to provide a self-healing coating that is effective for protecting molybdenum base and other refractory alloys from oxidation at elevated temper tures.

Another object of the invention is to provide a method for applying the aforesaid self-healing coatings to articles formed from molybdenum base alloys and other refractory metals.

Still another object of the invention is to provide an article of manufacture formed from a refractory metal and having especially good resistance to oxidation at high temperatures.

The above and other objects and features of the invention will become apparent from the following detailed description wherein the term molybdenum-base alloys means commercially pure molybdenum or any alloy of molybdenum which contains at least 50% molybdenum by weight and which oxidizes readily at elevated temperatures.

The coatings of the invention described herein all have a crystal structure of the spinel or perovskite type. The spinel structure is considered to be a variate atom equipoint framework where different atoms replace one another in structurally equivalent positions of a crystal. S'ructurally equivalent positions are areas in a unit cell that are related to other areas by the symmetry elecuts of the crystal lattice. Sixteen such equivalent positions exist to form structures of the type A3 0 where A is a divalent and B is a trivalent metal. The spinel structure consists of 32 oxygen ions and 34 metallic ions in a cubic unit cell where each A ion is tetrahedrally coordinated by 4, and each B ion is octahedrally coordinated by 6 oxygen neighbors. Each oxygen ion is bound to one A and three B ions. The structure is common to many mixed oxides where the ion A is divalent and the ion B is a trivalent metal. A structure of this type is extremely close packed and stable and thus, when bonded to the surface of a refractory metal article, prevents the diffusion of oxygen into the metal oxide interface. Studies on the mechanism of oxidation of molybdenum show that the oxidation reaction is initiated and proceeds at the metal oxide interface by the diffusion of oxygen ions inward through the initial molybdenum oxide. If an oxide such as a spinel having a close packed crystal structure can be bonded to the surface of the molybdenum or other refractory metal, such diffusion is effectively prevented.

As was stated above, the oxide bonded to the surface of molybdenum must be self-healing and must have a low vapor pressure at high temperatures to prevent evaporation of the coating. Taking these factors into consideration, the following spinel combinations are possible: beryllium aluminum oxide (BeAl O strontium aluminum oxide (SrAl O thorium strontium oxide (ThSr OQ, thorium beryllium oxide (ThBe O and molybdenum lithium oxide (MoLi -OQ. The application of the above combinations of oxides on refractory metals provides satisfactory coatings at high temperatures and does not impair the characteristics of the refractory metal.

The other close packed structure which may be used as a protective oxide coating for molybdenum and other refractory metals is the perovskite which corresponds to the composition A and has a cubic or pseudocubic unit cell, where A ions are situated at the corners of the cell and B ions at the center; and the faces are centered with oxygen. In this arrangement, there are six oxygen ions and nine metallic ions. Each A ion is in twelvefold and each B ion in sixfold coordination with oxygen neighbors. A pair of metallic ions that have radii appropriate to the coordination and an aggregate valency of six to confer electrical neutrality on the structure as a Whole should be able to form the perovskite structure The appearance of the perovskite structure with any pair of cations of the appropriate total charge illustrates a very important feature of ionic structures. In many such frameworks, the individual charges and exact distribution of the cations are of secondary importance when the conditions of electrical neutrality and geometric restrictions are satisfied.

The desirable perovskite combinations that can form on refractory metal surfaces, are self-healing, and have a low vapor pressure to prevent evaporation of the coating at high temperatures are: strontium silicon oxide (SrSiO silicon beryllium oxide (SiBeO thorium beryllium oxide (ThBeO yttrium aluminum oxide (YAlO and lanthanum aluminum oxide (LaAlO In order to form an effective coating for molybdenum and other refractory metals, there must be an inner coating of metallic cations which is covered by an outer oxide coating of the perovskite or spinel structure, the arrangement being such that if the outer coating ruptures, the metallic cations which become exposed to the atmosphere will oxidize to form a further perovskite or spinel oxide coating. It is in this Way that the coatings of the present invention are self-healing. If the molybdenum or other refractory metal is alloyed with the metal cations in the spinel or perovskite oxide coating, it is, in most cases, not necessary to apply an inner coating of these cations before applying the outer oxide since, if the oxide ruptures, the oxygen in the atmosphere will combine with the alloying elements in the molybdenum to again form an oxide coating.

Assuming that the refractory metal is not alloyed with the metallic cations of the perovskite or spinel coatings, it is necessary to first form on the finished machined or formed part a metallic case with a sub-oxide coating of the metallic cations. The procedure for the formation of this case follows the methods used in conventional case hardening processes. The molybdenum parts are exposed to a differential content of diffusion elements in a gaseous atmosphere or liquid metal or salt bath. In most cases the gaseous atmosphere will comprise organic or halide compounds of the metallic cations which make up the spinel or perovskite structure. After the organic or halide compounds are applied by gaseous diffusion, the coating is reduced or dissociated at elevated temperatures to leave behind the metallic cations and provided an alloy or chemical bond between the case and base metal. These cations are then oxidized under controlled oxidation conditions to form a sub-oxide of the cations on the case. The sub-oxides thus formed are deficient in oxygen and will, upon exposure to oxygen at elevated temperatures, form the completed spinel or perovskite structure. It will be apparent that control of the partial oxidation of the cations is necessary in this step in order to provide a basis for a chemical bond between the case and the final surface coating.

The case formed in the foregoing manner should be approximately 0.010 inch in thickness. After the case with its sub-oxide skin is formed on the surface of the refractory article, an inorganic compound of the spinel or perovskite type having a chemical composition related to its sub-oxide is applied to the case at room temperature by spraying, dipping, or slip casting. Finally, this inorganic compound is exposed to elevated temperatures below the recrystallization temperature of the refractory metal under a controlled oxidizing or neutral atmosphere. The outer coating applied in this manner should be ap proximately 0.003 inch thick.

The resulting coating on the refractory metal article is a covering consisting of the metallic case, the sub-oxide, and a perovskite or spinel layer. Thus, if the outer layer ruptures for some reason or other, the inner layer of the sub-oxide will combine with the oxygen in the atmosphere when the refractory metal is subjected to high temperatures to form a perovskite or spinel structure which prevents further diffusion of the oxygen inward to the base refractory metal,

As an example, if it is desired to protect a molybdenum base alloy with thorium-beryllium oxide, the molybdenum base article may be subjected to a gaseous atmosphere of thorium tetrachloride and beryllium chloride. There after, the surface of the molybdenum article is reduced or dissociated to leave behind free thorium and beryllium cations. These cations are then oxidized under controlled conditions to produce sub-oxides of thorium and beryllium. After the sub-oxides are thus formed, the perovskite and/ or spinel thorium-beryllium oxide is applied to the molybdenum article in any suitable manner. This final coat is then exposed to elevated temperatures under a controlled oxidizing or neutral atmosphere to produce a chemical bond between the outer spinel and/ or perovskite coating and the inner coating of sub-oxides. If thorium and beryllium were alloyed with the molybdenum, it would, of course, not be necessary to apply the original coating of thorium or beryllium cations since these elements are present in the alloy itself and will combine with oxygen.

From the foregoing it is apparent that the present invention provides a coating for refractory metals which is self-healing at elevated temperatures to prevent oxidation. The coatings produced in accordance with the invention are integrally bonded to the base metal and cannot be stripped mechanically from the body metal as is the case of coatings applied by electroplating or dipping. Furthermore, the coatings are inert with respect to the molybdenum base metal and show no evidence of reaction with the base metal after the final oxidation takes place. A distinct advantage of these coatings is that when the coatings form a discontinuous surface for one reason or another they develop a new self-healing surface from the elements of the case on the molybdenum article.

Although the invention has been described in connection with certain specific examples, it will be readily apparent to those skilled in the art that variou changes may be made to suit requirements without departing from the spirit and scope of the invention,

We claim as our invention:

1. An article of manufacture comprising a refractory metal base chemically bonded to a self-healing, oxidationinhibiting coating comprising an outer layer and an inner layer, said outer layer comprising at least one oxide selected from the group consisting of BeAl O SrAl O ThSr O ThBe O MoLi O SrSiO SiBeO ThBeO YA1O and LaAlO and said inner layer comprising at least one sub-oxide of the metallic cations of said oxide.

2. An article of manufacture in accordance with claim 1 wherein said refractory metal base contains a metallic case of the cations of said material.

3. An article of manufacture in accordance with claim 2 wherein said base comprises a refractory metal selected from the group consisting of molybdenum, niobium, vanadium, and base alloys thereof.

4. An article of manufacture in accordance with claim 1 wherein said base comprises an alloy selected from the group consisting of molybdenum, niobium and vanadium base alloys containing the metallic cations of said oxide.

5. In the method of applying to an article of molybdenum a self-healing, oxidation-resisting coating comprising an oxide selected from the group consisting of BeAl O SrAl O ThSr O ThBe O MoLi 0 SrSiO SiBeO ThBeO YAlO and LaAlO the steps of: subjecting the molybdenum article to a gaseous atmosphere containing compounds of the metallic cations of said oxide to thereby form a coating of the compounds on the article, treating the coating thus applied to leave said metallic cations on the surface of said article, thereafter oxidizing said cations under controlled conditions to form a sub-oxide layer thereof, applying said oxide to the suboxide layer thus formed, and heating the oxide thus applied at elevated temperatures to form a chemical bond with said sub-oxide layer.

6. In the method of applying to an article of refractory metal a self-healing, oxidation-resistant coating comp-rid ing an oxide selected from the group consisting of BeAl O SrAl O ThS1 O T hB O4, MOLl2O SrSiO SiBeO ThBeO YAlO and LaAlO the steps of: applying to the surface of said refractory metal article compounds of the metallic cations of said oxide to thereby form a coating of the compounds on the article, treating the coating thus applied to leave said metallic cations on the surface of said article, thereafter oxidizing said cations under controlled conditions to form a sub-oxide layer thereof, applying said oxide to said layer and heating the oxide thus applied at elevated temperatures to form a chemical bond with said layer.

7. A method of preventing high temperature oxidation of a refractory metal base comprising forming a case on said base, oxidizing said case to form a sub-oxide thereon, and applying thereover a coating comprising an oxide selected from the group consisting of BeAl O SI'A12O4, ThSr O ThBe O MOLizo SISiO SiBeO ThBeO (A and LaAlO said case comprising metallic cations of said oxide.

8. The method of claim 7 wherein the base comprises a refractory metal selected from the group consisting of molybdenum, niobium, vanadium, and base alloys thereof.

9. A method of preventing high temperature oxidation of a refractory metal base comprising forming on said base an inner layer comprising sub-oxides of metallic cations, and applying thereover an outer layer comprising an oxide selected from the group consisting of BeAl O Sl'A1 O4, ThSr O ThBfizO4, MOLi204, SrSiO SiBeO ThBeO YAlO and LaAlO said metallic cations being selected from the group consisting of the cations of said oxide.

10. The method of claim 9 wherein the refractory metal base comprises an alloy selected from the group consisting of molybdenum, niobium and vanadium base alloys containing the metallic cations of said oxide.

11. The method of preventing high temperature oxidation of a refractory metal base comprising exposing said base to a gaseous environment containing compounds of metallic cations for a time sufficient to form, after conversion of said compounds, a case on said base to a depth of about 0.010 inch, converting said compounds to form a case of metallic cations, treating said case under controlled oxidizing conditions to form thereon an inner layer comprising sub-oxides of said metallic cations, and applying over said inner layer an outer layer comprising an oxide selected from the group consisting of BeAl O SrAl O ThSr O ThBe O M05 0 SrSiO SiBeO ThBeO YAlO and LaAlO said metallic cations being selected from the group consisting of the cations of said oxide.

12. The method of claim 11 wherein said base comprises a refractory metal selected from the group consisting of molybdenum, niobium, vanadium and base alloys thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,015,172 Wiegand Sept. 24, 1935 2,424,645 Baumann et al July 29, 1947 2,538,959 Ballard Jan. 23, 1951 2,650,903 Garrison et al Sept. 1, 1953 2,665,474 Beidler et al Jan. 12, 1954 2,665,998 Campbell Jan. 12, 1954 2,707,157 Stanton Apr. 26, 1955 2,707,691 Wheildon May 3, 1955 2,775,531 Montgomery et a1. Dec. 25, 1956 2,788,290 Deuble Apr. 9, 1957 2,857,297 Moore et al Oct. 21, 1958 

5. IN THE METHOD OF APPLYING TO AN ARTICLE OF MOLYB-YBDENUM A SELF-HEALING, OXIDATION-RESISTING COATING COMPRISING AN OXIDE SELECTED FROM THE GROUP CONSISTING OF BE-AL2O, SRAL2O4. THSR2O4 THBE2O4 THBE2O4, MOLI2O4 SRSIO3, SIBEO3, THBEO3, YALO3 AND LAALO3, THE STEPS OF: SUBJECTING THE MOLYBDENUM ARTICLE TO A GASEOUS ATMOSPHERE CONTAINING COMPOUNDS OF THE METALLIC CATIONS OF SAID OXIDE TO THEREBY FORM A COATING OF THE COMPOUNDS ON THE ARTICLE, TREATING THE COATING THUS APPLIED TO LEACE SAID METALLIC CATIONS ON THE SURFACE OF SAID ARTICLE, THEREAFTER OXIDIZING SAID CATIONS UNDER CONTROLLED CONDITIONS TO FORM A SUB-OXIDE LAYER THEREOF, APPLYING SAID OXIDE TO THE SUBOXIDE LAYER THUS FORMED, AND HEATING THE OXIDE THUS APPLIED AT ELEVATGED TEMPERATURES TO FORM A CHEMICAL BOND WITH SAID SUB-OXIDE LAYER. 